In classical black-and-white photography a photographic element containing a silver halide emulsion layer coated on a transparent film support is imagewise exposed to light, producing a latent image within the emulsion layer. The film is then photographically processed to transform the latent image into a silver image that is a negative image of the subject photographed. Photographic processing involves developing (reducing silver halide grains containing latent image sites to silver), stopping development, and fixing (dissolving undeveloped silver halide grains). The resulting processed photographic element, commonly referred to as a negative, is placed between a uniform exposure light source and a second photographic element, commonly referred to as a photographic paper, containing a silver halide emulsion layer coated on a white paper support. Exposure of the emulsion layer of the photographic paper through the negative produces a latent image in the photographic paper that is a positive image of the subject originally photographed. Photographic processing of the photographic paper produces a positive silver image. The image bearing photographic paper is commonly referred to as a print.
In classical color photography in its most widely used form the photographic film contains three superimposed silver halide emulsion layer units each containing a different subtractive primary dye or dye precursor, one for recording blue light (i.e., blue) exposure and forming a yellow dye image, one for recording green exposure and forming a magenta dye image, and one for recording red exposure and forming a cyan dye image. During photographic processing developing agent is oxidized in the course of reducing latent image containing silver halide grains to silver, and the oxidized developing agent is employed to form the dye image, usually by reacting (coupling) with a dye precursor (a dye-forming coupler). Undeveloped silver halide is removed by fixing and the unwanted developed silver image is removed by bleaching during photographic processing. This approach is most commonly used to produce negative dye images (i.e., blue, green and red subject features appear yellow, magenta and cyan, respectively). Exposure of color paper through the color negative followed by photographic processing produces a positive color print.
Although widely used this form of classical color photography has evolved highly complicated complementary film and paper constructions. For example, a typical color negative film contains not only a minimum of three different emulsion layer units, but also dye-forming couplers, coupler solvents to facilitate their dispersion, masking couplers to minimize image hue distortions in printing onto color paper, and oxidized developing agent scavengers to avoid formation of unwanted dyes. Not only is the film structure complex, but the optical qualities of the film are degraded by the large quantities of ingredients related to dye image formation and management.
A much simpler film that has enjoyed commercial success in classical color photography is a color reversal film that contains three separate emulsion layer units for separately recording blue, green and red exposures, but contains no dye image forming ingredients. The film is initially processed like a black-and-white photographic film to produce three separate silver images in the blue, green and red recording emulsion layer units. The simplicity of construction has resulted in imaging properties superior to those of incorporated dye-forming coupler color negative films.
The factor that has limited use of these color reversal films is the cumbersome technique required for translating the blue, green and red exposure records into viewable yellow, magenta and cyan dye images. Three separate color developments are required to sequentially form dye images in the blue, green and red recording emulsion layer units. This is accomplished in each instance by rendering the silver halide remaining after black-and-white development developable in one layer and then employing a color developer containing a soluble dye-forming coupler to develop and form a dye image in one of the emulsion layer units. Developed silver is removed by bleaching to leave three reversal dye images in the photographic film.
In each of the classical forms of photography noted above the final image is intended to be viewed by the human eye. Thus, the conformation of the viewed image to the subject image, absent intended aesthetic departures, is the criterion of photographic success.
With the emergence of computer controlled data processing capabilities, interest has developed in extracting the information contained in an imagewise exposed photographic element instead of proceeding directly to a viewable image. It is now common practice to extract the information contained in both black-and-white and color images by scanning. The most common approach to scanning a black-and-white negative is to record point-by-point or line-by-line the transmission of a near infrared beam, relying on developed silver to modulate the beam. Another approach is to address areally the black-and-white negative relying on modulated transmission to a CCD array for image information recording. In color photography blue, green and red scanning beams are modulated by the yellow, magenta and cyan image dyes. In a variant color scanning approach the blue, green and red scanning beams are combined into a single white scanning beam modulated by the image dyes that is read through red, green and blue filters to create three separate records. The records produced by image dye modulation can then be read into any convenient memory medium (e.g., an optical disk). The advantage of reading an image into memory is that the information is now in a form that is free of the classical restraints of photographic embodiments. For example, age degradation of the photographic image can be for all practical purposes eliminated. Systematic manipulation (e.g., image reversal, hue alteration, etc.) of the image information that would be cumbersome or impossible to achieve in a controlled and reversible manner in a photographic element are readily achieved. The stored information can be retrieved from memory to modulate light exposures necessary to recreate the image as a photographic negative, slide or print at will. Alternatively, the image can be viewed as a video display or printed by a variety of techniques beyond the bounds of classical photography--e.g., xerography, ink jet printing, dye diffusion printing, etc.
A number of other film constructions have been suggested particularly adapted for producing photographic images intended to be extracted by scanning:
Kellogg et al U.S. Pat. No. 4,788,131 extracts image information from an imagewise exposed photographic element by stimulated emission from latent image sites of photographic elements held at extremely low temperatures. The required low temperatures are, of course, a deterrent to adopting this approach.
Levine U.S. Pat. No. 4,777,102 relies on the differential between accumulated incident and transmitted light during scanning to measure the light unsaturation remaining in silver halide grains after exposure. This approach is unattractive, since the difference in light unsaturation between a silver halide grain that has not been exposed and one that contains a latent image may be as low as four photons and variations in grain saturation can vary over a very large range.
Schumann et al U.S. Pat. No. 4,543,308 relies upon differentials in luminescence in developed color films to provide an image during scanning. Relying on differentials in luminescence from spectral sensitizing dye, the preferred embodiment of Schumann et al, is unattractive, since luminescence intensities are limited. Increasing spectral sensitizing dye concentrations beyond optimum levels is well recognized to desensitize silver halide emulsions.
Light reflection during imagewise exposure is a recognized phenomenon that is usually unwanted. When exposing light passes through an emulsion layer unit of a silver halide photographic element and is then reflected back so that it passes through the emulsion layer unit twice, the result is an unsharp image and the effect is referred to as halation, since a bright object will often appear to be surrounded by a halo. The common approach to reducing unwanted reflection is to incorporate in a photographic element an antihalation layer that absorbs exposing light after it has passed through the emulsion layer unit or units to prevent reflection. Antihalation layers are removed or decolorized during processing and therefore have no role in viewing the image. Typical antihalation materials are set out in Research Disclosure, Vol. 308, December 1989, Item 308119, Section VIII, paragraph C, and their discharge (decolorization or solubilization) is addressed in paragraph D. Research Disclosure is published by Kenneth Mason Publications, Ltd., Dudley House, 12 North St., Emsworth, Hampshire P010 7DQ, England.
While exposure reflection is undesirable in reducing image sharpness, it has been used to advantage to increase speed. Yutzy and Carroll U.K. Patent 760,775 disclose using titania or zinc oxide in an undercoat beneath a silver halide emulsion layer unit to reflect from 40 to 90 percent of the light received. Research Disclosure, Vol. 134, June 1975, Item 13452, discloses increasing photographic sensitivity by incorporating within or directly beneath an emulsion layer small reflective particles that scatter light. In FIG. 1 a relationship between particle size and light scattering is provided. Buhr et al Research Disclosure, Vol. 253, May 1985, Item 25330, discusses the transmission and reflection relationship between the thickness of tabular silver halide grains and the wavelength of light used for exposure.